This invention relates to an imaging medium and to a process for producing an image.
Images can be generated by exposing a photosensitive medium to light in an imagewise fashion. Some conventional non-silver halide photosensitive compositions contain molecules which are inherently photosensitive, so that absorption of electromagnetic radiation brings about decomposition of, at most, as many molecules as photons absorbed. However, a dramatic increase in the sensitivity of such photosensitive compositions can be achieved if the absorption of each photon generates a catalyst for a secondary reaction which is not radiation-dependent and which effects conversion of a plurality of molecules for each photon absorbed. For example, systems are known in which the primary photochemical reaction produces an acid, and this acid is employed catalytically to eliminate acid-labile groups in a secondary, radiation-independent reaction. Such systems may be used as photoresists: see, for example, U.S. Pat. Nos. 3,932,514 and 3,915,706; and Ito et al., "Chemical Amplification in the Design of Dry Developing Resist Materials, Polym. Sci. Eng., 23(18), 1012 (1983).
Among the known acid-generating materials for use in this type of process employing secondary, non-radiation dependent reactions are certain diazonium, phosphonium, sulfonium and iodonium salts. These salts, hereinafter referred to as superacid precursors, decompose to produce superacids, i.e., acids with a pKa less than about 0, upon exposure to electromagnetic radiation. Other materials decompose to produce superacids in a similar manner. However, in the absence of a spectral sensitizer, the known superacid precursors decompose to produce superacid only upon exposure to wavelengths which the precursors absorb, which are typically in the short ultraviolet region (below about 280 nm). The use of such wavelengths is often inconvenient, not least because special optical systems must be used.
It is known that various dyes can sensitize the decomposition of superacid precursors upon absorption by the dye of radiation which is not significantly absorbed by the superacid precursor; see, for example, European Patent Application Publication No. 120,601. Unfortunately, however, due to the very low pKa of the superacid, many such dyes are protonated by the superacid, so that no unbuffered superacid is produced (i.e., the sensitizing dye buffers any superacid produced). Since no unbuffered superacid is released into the medium, decomposition of superacid precursors sensitized by these dyes cannot be used to trigger any secondary reaction which requires the presence of unbuffered superacid.
(The term "unbuffered superacid" is used herein to refer to superacid which is not buffered by the sensitizing dye, and which thus provides an acidic species stronger than that provided by the protonated sensitizing dye. Because of the extreme acidity of superacids and their consequent tendency to protonate even species which are not normally regarded as basic, it is possible, and indeed likely, that "unbuffered superacid" will in fact be present as a species buffered by some component of the imaging medium less basic than the sensitizing dye. However, such buffering by other species may be ignored for the present purposes, so long as superacid is present as an acidic spedes stronger than that provided by superacid buffered by the sensitizing dye.)
Crivello and Lam, "Dye-Sensitized Photoinitiated Cationic Polymerization", J. Polymer Sci., 16, 2441 (1978) and Ohe and Ichimura, "Positive-Working Photoresists Sensitive to Visible Light III, Poly(tetrahydropyranyl methacrylates) Activated by Dye-Sensitized Decomposition of Diphenyliodonium Salt", J. Imag. Sci., Technol., 37(3), 250 (1993) describe small sub-groups of sensitizing dyes which are sufficiently non-basic that the buffered superacids produced can effect certain acid-catalyzed reactions. However, the need to restrict the choice both of sensitizers and of acid-catalyzed reactions may make it difficult to design an efficient imaging system at a specific desired wavelength.
A variety of non-basic, polycyclic aromatic compounds sensitize decomposition of superacid precursors to produce unbuffered superacid upon exposure to longer wavelengths than the superacid precursors absorb themselves. Such materials are discussed in, for example, DeVoe et al., "Electron Transfer Sensitized Photolysis of 'Onium salts", Can. J. Chem., 66, 319 (1988); Saeva, U.S. Pat. No. 5,055,376; and Wallraffet al., "A Chemically Amplified Photoresist for Visible Laser Imaging", J. Imag. Sci. Technol., 36(5), 468-476 (1992).
U.S. Pat. No. 5,286,612 (assigned to the same assignee as the present invention) describes a process by which a wider variety of dyes than those discussed above may be used together with a superacid precursor to generate free (unbuffered) superacid in a medium. In this process, acid is generated by exposing a mixture of a superacid precursor and a dye to actinic radiation of a first wavelength which does not, in the absence of the dye, cause decomposition of the superacid precursor to form the corresponding superacid, thereby causing absorption of the actinic radiation and decomposition of part of the superacid precursor, with formation of a protonated product derived from the dye; then irradiating the mixture with actinic radiation of a second wavelength, thereby causing decomposition of part of the remaining superacid precursor, with formation of free superacid. Generation of superacid by exposure to the second wavelength may be sensitized by one of the non-basic, polycyclic aromatic sensitizers mentioned above. (For convenience, the type of process disclosed in this patent will hereinafter be called the '612 process.)
However, the use of any of the methods described above for the sensitized decomposition of superacid precursors to generate unbuffered superacid poses a severe problem if the exposing wavelength for the imaging system falls within a wavelength range in which the resultant image is to be viewed, and the sensitizing dye is not removed. This problem will hereinafter be referred to as the "sensitizing/viewing problem," and is particularly apparent when one attempts to use one of the sensitizers described above to make an image, intended to be viewed by the eye, by means of an exposure to visible wavelengths. Absorption of visible light by the sensitizer leads to a large minimum optical density (D.sub.min) in the final image, lowering its contrast and making its appearance unacceptable (especially in regions intended to be white, that is diffusely reflective, and non-absorbing in the visible region). Even if the visible absorption of the sensitizer is bleached in the course of the sensitization reaction, this visible absorption will still remain in the originally non-exposed areas of the image. The sensitizing/viewing problem is not confined to visible wavelengths but applies to any system in which the exposing radiation absorbed by the sensitizer falls within a wavelength range in which the resultant image is to be viewed or used (in the case of a photomask or photographic negative). For example, in the graphic arts industry, it is conventional to expose contact and "dupe" films in the near ultra-violet to produce photomasks which display imagewise changes in absorption which are themselves in the near ultra-violet range. Such processes, if sensitized by one of the methods described above, may suffer from a sensitizing/viewing problem.
The sensitizing/viewing problem is especially severe if it is desired to construct a full-color imaging system, which requires exposure to at least three different wavelengths to produce images in three primary colors. A conventional approach to this problem would be to resort to "false sensitization", i.e., to expose the imaging medium at three wavelengths which are not visible to the eye. In practice, however, exclusion of visible wavelengths for exposure leads to great difficulty.
Efficient sensitization of superacid precursors at non-visible wavelengths is limited to the near ultra-violet region (between about 330 nm and 420 nm) and the near infra-red region (between about 700 nm and 1200 nm). The non-basic, polycyclic aromatic compounds mentioned above may be used to sensitize decomposition of superacid precursors to make unbuffered superacid in the near ultra-violet region. However, it is difficult to find three near ultra-violet wavelengths which can be generated by a convenient source and which are sufficiently separated from each other to be absorbed by three sensitizers without cross-talk among three color-forming layers. (The practical near ultra-violet wavelength range for this type of imaging process extends only from about 330 nm to about 420 nm, since other components of the imaging medium, such as leuco image dyes if used, often absorb competitively with the sensitizer below about 330 nm, and wavelengths above about 420 nm will appear yellow to the human eye). It is difficult to find a lamp which emits three spaced wavelengths in the 330-420 nm range; a conventional mercury lamp emits only two usable wavelengths in this range. Moreover, a modulating device such as a liquid crystal cell (and polarizers, if required) may be damaged by lengthy exposure to near ultra-violet radiation. Although phosphors emitting in the near ultra-violet are known, cathode ray tubes using such phosphors must be custom-made, and the emission spectra of such phosphors is often broad, leading to possible cross-talk. Another available exposure source, the near ultra-violet laser, is expensive.
Although it may at first appear that sensitization at near infra-red wavelengths would solve the problems discussed above, infra-red sensitizers suffer from other problems. Some near infra-red dyes, and all non-basic polycyclic aromatic compounds which absorb in the near infra-red, have visible absorptions which contribute to the D.sub.min of the image. Equally importantly, the photon energy in the near infra-red is only about 35 kcal/mole, and in practice this low photon energy appears to limit the quantum efficiency at which the sensitizing dye causes the decomposition of the superacid precursor. This limited quantum efficiency limits the overall sensitivity of the process.
The problems discussed above are alleviated if at least one of the exposing wavelengths is in the visible region. If at least one exposing wavelength is in the visible or near infra-red region, a maximum of only two exposing wavelengths must be found in the near ultra-violet. Furthermore, the higher photon energy available in the visible region, as compared with the near infra-red, is likely to lead to higher quantum efficiency, and therefore higher sensitivity, if the imaging medium is exposed in the visible, rather than the near infra-red. There are also other reasons why exposure in the visible region is preferred. Conventional cameras are designed to use visible light, and thus any medium intended to replace conventional silver halide film in cameras and produce a direct positive print or transparency must use visible light to produce a visible image. Likewise, any medium intended to produce a print from a photographic negative must be capable of exposure by visible light. Conventional photographic printers, also, are designed to use visible light. Much effort has been expended in developing sources (for example, light-emitting diodes, cathode ray tubes, lamps and lasers) and control devices (for example, liquid crystal light valves) optimized for visible radiation. Accordingly, using visible radiation rather than (say) infra-red or ultra-violet radiation in an imaging process often enables the cost of the imaging apparatus to be reduced, and may also make it possible to use a standard, commercially available light source or control device rather than a custom-made source or device.
Copending U.S. application Ser. Nos. 07/965,162 (now U.S. Pat. No. 5,334,489) and 08/141,860 (both of which are assigned to the same assignee as the present application), and the corresponding International Application No. PCT/US93/10224 (Publication No. WO 94/10607), all describe processes for the photochemical generation of acid and for imaging using conventional ultra-violet sensitizers; these processes will hereinafter collectively be called the "'860 process." The aforementioned U.S. Pat. No. 5,286,612 and copending application Ser. No. 08/141,852 (both of which are assigned to the same assignee as the present application), and the corresponding International Application No. PCT/US93/10215 (Publication No. WO 94/10606), all describe an imaging process using an imaging medium comprising an acid-generating layer or phase and a color-change layer or phase. In this patent and these applications, the acid-generating layer or phase comprises a mixture of a superacid precursor, a sensitizer and a secondary acid generator. The secondary acid generator is capable of acid-catalyzed thermal decomposition by unbuffered superacid to form a second acid. The color-change layer comprises an image dye which undergoes a change in its absorption of radiation upon contact with the second acid. After imagewise exposure and generation of unbuffered superacid in the exposed areas, the medium is heated; in exposed areas, the unbuffered superacid causes acid-catalyzed decomposition of the secondary acid generator, thereby causing the formation of a molar amount of second acid much larger than the molar amount of unbuffered superacid present before the heating. In the nonexposed areas, however, since no unbuffered superacid is present, no significant generation of second acid takes place during the heating. Thereafter, the medium is further heated (in practice the two heating steps can be combined) to cause the components present in the two layers to mix, so that, in exposed areas, the second acid brings about the absorption change in the image dye, thereby forming an image. Thus, the imaging medium is a single sheet which develops its image without any need for treatment with a developing composition and without requiring any waste material to be peeled from the medium to produce the final image.
In these processes, the sensitizer or sensitizers and the undecomposed superacid precursor remain present in at least the non-exposed areas of the medium after imaging (these non-exposed areas are the D.sub.min areas, i.e., the regions of minimum optical density, in the normal case where the image dye is colorless before imaging and develops color in the exposed areas during imaging). Accordingly, in practice it is necessary that the image be viewed in a wavelength range which does not include the wavelength used for the imagewise exposure, since otherwise the image will suffer from a sensitizing/viewing problem, with the contrast between the exposed and non-exposed areas reduced by the absorption, in the non-exposed areas, of the sensitizer or sensitizers used. For example, as discussed above, the preferred '612 process requires two exposures, one at a wavelength in the near infra-red region (700-1200) nm and the second at a wavelength in the near ultra-violet region, typically around 365 nm, to produce an image in the visible region (about 400-700 nm).
There is a need for a modified form of these imaging media and processes which removes the restriction that the wavelength used for the imagewise exposure be outside the wavelength range in which the final image is viewed.